1. Field of the Invention
The present invention relates to an induction heating cooking apparatus. More specifically, the present invention relates to an induction heating cooking apparatus employing such an element as a transistor, a gate turn-off thyristor or the like as a switching element for a self-controlled inverter.
2. Description of the Prior Art
An induction heating cooking apparatus is provided with a ripple current voltage obtained by rectifying the commercial alternating current source voltage or a direct current voltage obtained by smoothing the above described ripple current voltage. An inverter is energized by the ripple current voltage or the direct voltage, so that the inverter makes a high frequency oscillation of about 20 to 40 kHz. The inverter is implemented with an induction heating coil, so that a high frequency alternating magnetic field is generated by the induction heating coil in accordance with the high frequency oscillation of the inverter. A cooking pan made of an iron group metal, such as iron, stainless steel, or the like, disposed in the vicinity of the induction heating coil is induction heated by the high frequency alternating magnetic field. An example of such an induction heating cooking apparatus is disclosed in, for example, U.S. Pat. No. 3,781,503, issued Dec. 26, 1973 to Harnden, Jr. et al. and entitled "SOLID STATE INDUCTION COOKING APPLIANCES AND CIRCUITS". An inverter of such an induction heating cooking apparatus comprises a switching element connected to the induction heating coil. As a switching element, the above referenced U.S. Pat. No. 3,781,503 employs a silicon controlled rectifier. However, in the case where a silicon controlled rectifier is employed as a switching element, it is necessary to separately provide a turning-off circuit for the silicon controlled rectifier, which makes the circuit configuration complicated.
Accordingly, of late it has been proposed that as such a switching element, a switching element for a simple control of conduction or non-conduction, such as a transistor of a relatively large capacity, a gate turn-off thyristor, or the like, be used in place of a silicon controlled rectifier. An inverter employing such a switching element is implemented as a self-controlled oscillation type. A self-controlled oscillation inverter requires a signal for continuing oscillation of the inverter responsive to the output of the inverter after the inverter is once triggered, i.e. control means for controllably rendering the switching device conductive or non-conductive.
An inverter employing such control means as described above is disclosed in, for example, U.S. Pat. No. 4,115,676 issued Sept. 19, 1978 to Higuchi et al. and entitled "INDUCTION HEATING APPARATUS". The above described U.S. Pat. No. 4,115,676 is the prior art of most interest to the present invention. A self-excited inverter of U.S. Pat. No. 4,115,676 comprises an induction coil, a transistor connected in series therewith, a resonance capacitor connected in parallel with the transistor and a flywheel diode. The inverter is energized with a direct current voltage obtained by full-wave rectifying the commercial power supply voltage. A current flowing through the induction coil is detected between the rectifying circuit and the induction coil. More specifically, a coil for detecting a current flowing through the current path of the induction coil is coupled to the rectifying circuit side. The base electrode of the transistor is connected to receive a signal for controlling the transistor to be conductive or non-conductive in accordance with the current detected from the coil. However, such an inverter as disclosed in U.S. Pat. No. 4,115,676, which controls the transistor based on the current flowing through the induction coil, involves a problem to be set forth in the following. More specifically, the current flowing through the induction coil is not necessarily the same as the current flowing through the switching transistor. The current flowing through the transistor is not related to the current flowing through the induction coil, but could involve a surge current, a vibration or the like. In other words, detection of the current flowing through the induction coil does not necessarily result in accurate detection of the current of the transistor caused by such a surge current, a vibration or the like. Accordingly, control of such a transistor based on the current flowing through the induction coil is not sufficient to protect the switching transistor. More specifically, since such a surge current is not detected as a current in the induction coil, even if the surge current flows through the transistor, the transistor can not be effectively prevented from being damaged with such a surge current.